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For an isobaric process, we wish to use the Enthalpy, for the pressure appears in the differential there. This choice allows us to carefully account for the work done on the system and that done on the environment in a process.

For a process that is both isobaric and isothermal, it makes sense to look at the Gibbs Free Energy.

Therefore, while solving problems, look for what is being held constant so that you can make an appropriate choice of energy.